Using non-Tumbleweed repositories with Tumbleweed?

I am a long time openSUSE user very seriously considering changing from Leap 42.3 to Tumbleweed for the very first time, via a fresh install.

Can non-Tumbleweed, stable repositories be successfully used with Tumbleweed? For example, could I use these stable repos with Tumbleweed:

Are there any problems caused by using non-Tumbleweed repos with Tumbleweed? If yes, please elaborate.

You should not mix repos for different versions of openSUSE.

No. You’d bork your system. Even the 42.1 repo for virtualbox is not OK. It maybe working today but since 42.1 is out of support and maintenance, this will break,
If you want to use the content of these repos in a TW version, browse for TW’s equivalents or rather first check https:/ for availability in TW.

So, you are saying everyting not in Tumbleweed repos, including popular applications like VirtualBox from Oracle’s servers, is forbidden?

I do not say it is forbidden. You can install what you want on systems you own. I do not care.

I understood the question as: “Is it wise to do so (mixing software from repos labeled as created for other openSUSE versions) when I want to keep a well running TW system at the same time”.

In short:: Use At Own Risc. Forbidden is not the word.

A bit more: openSUSE has absolutely no control over 3rd party packages, openSUSE is dedicated to the FOSS principles, so it will f.e. deliver the FOSS Vbox packages, but for f.e. nvidia driver packages one needs to activate a 3rd party repo.

Some advice: IME sticking to only TW repos ( and as few as possible ) is the best way to go if you want a stable system.

I did not exactly intend the meaning of “forbidden”, and could not think of a better word for it at the time. You correctly understood my intent anyway.